This invention relates to novel coating compositions and more specifically to corrosion-resisting coatings which can be applied directly to various surfaces particularly metal either as a high or low gloss, self-priming topcoat.
Various surfaces and particularly metal surfaces require the protection of coatings especially when the surfaces are exposed to a corrosive environment. Metal surfaces of aircraft, for example, are exposed to seawater which require protection from corrosion. Specifically, aircraft, e.g., Navy aircraft, are exposed to seawater spray in addition to various acid-forming gases such as sulfur dioxide, carbon dioxide, etc. Moreover, in addition to aircraft, various machinery and equipment in the industrial environments, where fossil fuels are used need protection against corrosion. It is important therefore that the coating be resistant to corrosion, various chemicals, the weather and at the same time be flexible and have good adhesion to the substrates.
Presently, coating systems comprise one or more films, i.e., an undercoat, a sealant and a topcoat. Aircraft, for example, traditionally have been coated with high performance two-component protective films consisting of an epoxy primer and a polyurethane topcoat. The type of epoxy primers used on the aircraft are designed to adhere to the metal surface and help to improve the adhesion of the topcoat and together prevent corrosion of the metal. However, these undercoats require a topcoat, since the undercoats lack flexibility especially at low temperatures (-60.degree. F.) resulting in extensive cracking particularly in highly flexed areas of the aircraft. Moreover, the undercoats usually lack weather resistance and generally cannot be formulated in different colors required for aircraft.
In comparison, the polyurethane coatings of this invention, provides the necessary corrosion resistance, the required degree of flexibility, the desired optical properties, and a resistance to weather and various chemicals. To obtain these characteristics, the multi-film coatings used heretofore generally required a dry-film thickness ranging up to about 0.005 inches, e.g., up to about 10 nils or more which added considerable weight to the aircraft. In addition, the multi coats are time consuming to apply particularly since there is a drying time between each application. Further, the removal of a two-coat system can be difficult and time consuming and generates high levels of volatile organic (VOC) emissions during the operations.
In accordance with this invention the corrosion-resistant coating comprise a polyurethane binder derived from the reaction of at least one polyol, e.g., polyester polyol and an isocyanate, e.g., hexamethylene diisocyanate (HDI) or polyisocyanates in combination with a unique mixture of corrosion-inhibiting pigments consisting essentially of (1) an alkaline earth metal phosphate, e.g., zinc-barium phosphate, (2) zinc salts of benzoic acid or substituted benzoic acid, and (3) molybdate-modified zinc phosphates. All three of these compounds are essential, in the stated relative proportions, to provide a single high-gloss coating with the necessary corrosion resistance and adhesion characteristics required of a good top coat. In addition, titanium dioxide (TiO.sub.2) including spherical TiO.sub.2 particles, e.g., vesiculated beads of TiO.sub.2 are included as a pigment together with these three alkaline earth metal or zinc salts. The coating compositions of this invention may be applied, as one coat, directly onto various hard surfaces such as metal and/or organic matrix composites, etc., and do not require an undercoat to provide a corrosion-resistant finish with desired optical and adhesion properties.